1.2- dioxetane, fragmentation

Scheme 8.4. Dioxetane fragmentation initiated by enzymatic ester hydrolysis.

Catalytic reductions over platinum or palladium, which are usually quantitative methods for the identification of organic peroxides, are problematic. Little of the expected 1,2-diol is obtained because the dioxetane fragments into its carbonyl products due to metal catalysis." However, lithium aluminium hydride reduction under subambient conditions affords the expected 1,2-diol quantitatively. Again, the sterically hindered dioxetane (9) is an exception. Here zinc in acetic acid proved successful. ... 

Two extreme mechanisms have been proposed for the unimolecular dioxetane decomposition the concerted mechanism , whereby cleavage of the peroxide and the ring C—C bond occurs simultaneously, and the biradical mechanism whereby the initial cleavage of the 0—0 bond leads to the formation of a 1,4-dioxy biradical whose subsequent C—C bond cleavage leads to the formation of the two carbonyl fragments (Scheme 8). Although the biradical mechanism adequately explains the activation parameters obtained for most of the dioxetanes smdied, it appears not to be the appropriate mechanistic model for the rationalization of singlet and triplet quanmm yields. Therefore, an intermediate mechanism has been proposed, whereby the C—C and 0—0 bonds cleave in a concerted, but not simultaneous, manner (Scheme 8) . 

Like other peroxides, also dioxetanes are sensitive to the presence of metal ions and their complexes, which catalyze the decomposition of the dioxetane molecule. In most cases, this decomposition is dark, i.e. no chemiluminesce is generated in such a catalytic cleavage42. An informative exception, for instance, constitutes the chemiluminescent decomposition of the dioxetane 19 in Scheme 13, initiated by the ruthenium complex Ru(bipy)3Cl243. It has been shown that this chemiexcitation derives from the valence change of the ruthenium ion in the process Ru3+ I e — Ru2+, for which the efficiency of the excited-state generation may be as much as 40%44. Hence, when the radical anion of the carbonyl cleavage fragment from the dioxetane and the Ru3+ ion are formed in... 

With electron rich olefins 1,2 cycloaddition forms relatively unstable dioxetanes which cleave to give carbonyl fragments ... 

Kinetically stabilized azetes also show a high tendency for cycloaddition with a variety of other reagents. Cycloaddition of (47) with triplet oxygen produced a fully characterized dioxetan adduct (48), which decomposed at 25°C into t-butyl cyanide and the a-dione fragment (88AG(E)272). 

The nature of the adduct formed according to Eq. (19) depends on the structure of the donor. When the donor is a monoolefin, the initial product, a dioxetane, may fragment into two ketones [143]. On the other hand, several substrates giving rise to bifunctional radical cations are trapped to form more or less stable adducts. Thus, tetraphenyloxirane gave rise to an ozonide (15) [149], several 1,4-bifunctional radical cations formed dioxanes (16) [150], and diolefms formed dioxenes (17) [151]. 

As highlighted in Section 2.16.2, simple alkyl-substituted 1,2-dioxetanes are thermally labile compounds and decompose through a twisted diradical-like transition state to afford two carbonyl fragments one of which is predominantly a triplet-excited carbonyl (Scheme 3). Activation barriers are often in the order of 25 kcal mol. ... 

Besides the thermal C-C decomposition of dioxetanes into carbonyl fragments, C-O bond cleavage is occasionally seen for specific substrates to afford allylic hydroperoxides (Equation 1) <2004JA16777>. 

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